The present invention generally relates to high frequency, high data rate communication systems and, more particularly, to demodulation for band efficient quadrature phase shift keying (QPSK) modulation and quadrature amplitude modulation (QAM) using monolithic microwave integrated circuits (MMIC).
The present invention is applicable to bandwidth efficient modulation communication systems. The invention provides a non-wired approach for high data rate needs, for example, in satellite to satellite communication, satellite to ground communication, terrestrial relay links for line-of-sight, demod/remod systems, and space based demod/remod systems.
The purpose of a demodulator is to perform waveform recovery. The best demodulator will recover the baseband pulse with the best signal to noise ratio (SNR). The error degradation in the signals can be caused by two most prominent sources. One most prominent source is inter-symbol interference (ISI). The filtering used to reject unwanted parts of the signal and noise can cause a non-ideal system transfer causing ISI. The ISI distorts the signal and will produce errors in the received signal. The other most prominent source of degradation is due to noise from electrical sources, atmospheric effects, thermal effects, and inter-modulation products, for example. The demodulator should undistort, or correct distortion of, the pulse to give the best possible received signal.
Conventional modulation systems consist of a modulator operated at an intermediate frequency (IF) and a number of filters, amplifiers and mixers that up convert the modulated signal to the transmit frequency, also called the carrier frequency. The wide band signal on the carrier is transmitted over a communication channel and received by a receiver. At the receiver, the wide band signal on the carrier is down converted to an IF channel and then demodulated. The IF channel may be optimized for the control of noise sources to allow increases in data rates and improve link margins. Down conversion of the wide band signals to optimized IF channels may cause phase errors to be introduced into the data that induce increases in the bit error rate of the communication channel. A significant problem for bandwidth efficient modulation and demodulation is achieving low amplitude and phase error.
An example of QPSK modulation is illustrated by the block diagram of FIG. 1, where two-bit data word 102 and carrier 104 are input to phase modulator 100, which outputs QPSK modulated carrier 106 corresponding to a signal, S, of the form:
S(t)=A cos(xcfx89stxe2x88x92xcex8+"psgr")xe2x80x83xe2x80x83(1) 
where A is the carrier amplitude constant and "psgr" is the phase constant. There are four possible values for two-bit data word 102. Phase modulator 100 maps each of the four possible values for two-bit data word 102 to a distinct value of the phase angle xcex8.
The QPSK modulated carrier 106 output from phase modulator 100 may be represented on a phase diagram such as phase diagram 200 seen in FIG. 2. Phase diagram 200 shows that phase angle xcex8 will take on the form of one of four phases separated by 90 degrees. As shown in FIG. 2, each of the four possible values of two-bit data word 102 is represented by a symbol 202, which is a point, or vector, s1, s2, s3 or s4, in the phase plane of phase diagram 200. Two bits of information, or one symbol, is sent every word time corresponding to one of the four vectors, or symbols, in phase diagram 200.
The symbols of a QPSK signals may also be conceptualized as two pairs of a bi-orthogonal set. FIG. 3 shows a common implementation, using that concept, of QPSK modulator 300 employing orthogonal bi-phase shift keying (BPSK) modulators 310 and 320. The circuit of QPSK modulator 300 shown in FIG. 3 uses double-balanced mixers for BPSK modulators 310 and 320. As seen in FIG. 3, two-bit data word 302 is extracted from bit sequences 303 and 305. Bit sequence 303 and carrier 314 are input to BPSK modulator 310, which outputs BPSK modulated signal 316. Bit sequence 305 and carrier 324 are input to BPSK modulator 320, which outputs BPSK modulated signal 326. BPSK modulated signals 316 and 326 are added by summer 330 and output as QPSK modulated carrier 336 corresponding to a signal, S, of the form:
S(t)=A cos(xcfx89stxe2x88x92xcex8+"psgr")xe2x80x83xe2x80x83(2) 
where A is the carrier amplitude constant and "psgr" is the phase constant. There are four possible values for two-bit data word 302 each of which is mapped to a distinct value of the phase angle xcex8. Because carriers 314 and 316 differ in phase by 90 degrees, phase angle xcex8 will take on one of four phase values separated by 90 degrees, as shown in FIG. 2, with each of the four possible values of two-bit data word 302 represented by a symbol 202, which is a vector, s1, s2, s3 or s4, in the phase plane of phase diagram 200.
FIG. 4 shows how two QPSK modulation systems 410 and 420 may be combined in a QAM modulation system 400 to achieve a 16 QAM signal 436. A radio frequency (RF) or IF carrier is provided by local oscillator 404 using timing reference 401, as known in the art. The RF or IF carrier is split into carriers 414 and 424, and each is fed into QPSK modulation systems 410 and 420, respectively. Two-bit data word 412, which includes bits b0 and b1 as shown in FIG. 4, and carrier 414 are input to QPSK modulation system 410. QPSK modulation system 410 outputs QPSK modulated carrier 416 corresponding to a signal which may be represented, as described above in connection with FIG. 2, by vectors 516 on phase diagram 510 shown in FIG. 5. Similarly, two-bit data word 422, which includes bits b2 and b3 as shown in FIG. 4, and carrier 424 are input to QPSK modulation system 420. QPSK modulation system 420 outputs a QPSK modulated carrier 426, which travels through attenuator 427. Attenuator 427 lowers the amplitude of QPSK modulated carrier 426. The attenuated QPSK modulated carrier 426 corresponds to a signal which may be represented, as described above in connection with FIG. 2, by vectors 526 on phase diagram 520 shown in FIG. 5.
As seen in FIG. 4, the two QPSK modulated carriers 416 and 426 are added by summer 430 and output as QAM modulated carrier 436 corresponding to a signal which may be represented, as described above in connection with FIG. 2, by vectors 536 on phase diagram 530 shown in FIG. 5. The addition of QPSK modulated carriers 416 and 426 is indicated in FIG. 5 by plus sign 532 and equal sign 534 representing addition of phase diagrams 510 and 520 corresponding to QPSK modulated carriers 416 and 426, respectively. Because each vector 516 and 526 represents a signal, addition of the phase diagrams is accomplished by adding each possible pair of vectors 516 and 526 to produce a vector or symbol 536 in phase diagram 530. The configuration formed by symbols 536 is referred to as a 16 QAM constellation. The vectors 516 are also shown in phase diagram 530 to provide a size orientation for the purposes of illustration only, but do not form part of the 16 QAM constellation illustrated in phase diagram 530. Each symbol 536 represents a pair of two-bit data words 412 and 422, which may be viewed as a four-bit data word, b0, b1, b2, b3. Each four-bit data word has 16 possible values each of which is mapped by QAM modulation system 400 to one distinct symbol 536 of the 16 symbols 536.
Physical limitations and variances in the circuits used to implement QAM modulation system 400 cause variance, or inexactitude, in the amplitudes and phases of symbols 536 during transmission of the QAM modulated signal. The variances may cause some of the symbols 536 to occasionally be transmitted closer together in phase diagram 530. In other words, the amplitude and phase of two different symbols could begin to overlap, which may be referred to as inter-symbol interference. If the variances, or inter-symbol interference, are too great, the receiver may not be able to reliably provide resolution of the symbols to permit faithful demodulation of the QAM modulated signal at the receiver. Inter-symbol interference, which limits the effectiveness of QAM modulation systems, may be reduced by achieving low amplitude and phase error over the channel bandwidth. The technical difficulties in achieving low amplitude and phase error over the channel bandwidth have restricted the use of QAM modulation systems to lower frequencies and narrower bandwidths than is desirable. Introduction of amplitude and phase error is a significant problem in the demodulation of signals as well as in the modulation of signals, and, for example, may be associated in the receiver, as described above, with the down conversion of signals to IF.
As can be seen, there is a need in communication systems for demodulation that reduces amplitude and phase error, thus allowing higher data rates relative to the carrier frequency. There is also a need in communication systems for demodulation that achieves low amplitude and phase error over a wide bandwidth, thus improving overall performance by lowering the bit error rate of the communication channel.
The present invention provides, in communication systems, demodulation that reduces amplitude and phase error, and allows higher data rates relative to the carrier frequency. The present invention also provides, in communication systems, demodulation that achieves low amplitude and phase error over a wide bandwidth, and improves overall performance of the communication system measured by the bit error rate of the system.
In one aspect of the present invention, a system for recovering and demodulating a carrier includes a carrier recovery loop, which receives the carrier as an input and produces a recovered carrier at a frequency approximately equal to that of the carrier; and a data detector, which receives the carrier and the recovered carrier as inputs and uses the recovered carrier to demodulate the carrier and detect I channel data and Q channel data.
In another aspect of the present invention, a system for recovering and demodulating a carrier includes a carrier recovery loop and a data detector. The carrier recovery loop receives the carrier as an input and produces a recovered carrier at a frequency approximately equal to that of the carrier. The carrier recovery loop includes a downconverting mixer, a xc3x975 multiplier, a xc3x974 multiplier, and a phase locked loop. The downconverting mixer receives the carrier input, and the phase locked loop provides a VCO reference frequency through the xc3x975 multiplier to the downconverting mixer, which provides a frequency shifted signal. The frequency shifted signal is passed through the xc3x974 multiplier as input to the phase locked loop. The data detector receives the carrier and the recovered carrier as inputs and uses the recovered carrier to demodulate the carrier and detect I channel data and Q channel data.
In still another aspect of the present invention, a system for recovering and demodulating a carrier includes a carrier recovery loop, which receives the carrier as an input and produces a recovered carrier at a frequency approximately equal to that of the carrier. The carrier recovery loop includes a downconverting mixer, a xc3x975 multiplier, a xc3x974 multiplier, and a phase locked loop. The downconverting mixer receives the carrier as one input, and the phase locked loop provides a VCO reference frequency through the xc3x975 multiplier to the downconverting mixer as the other input. Using the VCO output to do the downconversion reduces the number of sources of phase noise and the VCO output is coherent so the amount of phase noise is reduced. The VCO reference frequency is approximately one quarter the carrier frequency, and the xc3x975 multiplier upconverts the VCO reference frequency by a factor of five and feeds the upconverted VCO reference frequency as the other input to the downconverting mixer. Based on its inputs, the downconverting mixer provides a frequency shifted signal, whose frequency is approximately one quarter the carrier frequency, which is passed through the xc3x974 multiplier. The xc3x974 multiplier upconverts the frequency shifted signal by a factor of four and feeds the upconverted frequency shifted signal as input to the phase locked loop.
The phase locked loop includes a phase detector, a loop filter, a VCO, and another xc3x974 multiplier. The VCO provides the VCO reference frequency to the phase locked loop""s xc3x974 multiplier, which produces an upconverted VCO reference frequency to provide the recovered carrier. The phase detector compares the upconverted frequency shifted signal to the upconverted VCO reference frequency in order to drive a loop filter, which provides a control voltage to the VCO.
The system also includes a phase adjuster. The recovered carrier is passed through the phase adjuster to provide a phase adjusted recovered carrier. The phase adjuster matches the phase of the phase adjusted recovered carrier to the phase of the carrier.
The system lastly includes a data detector, which receives the wideband carrier signal and the phase adjusted recovered carrier as inputs and uses the phase adjusted recovered carrier to demodulate the carrier and detect I channel data and Q channel data. The wideband carrier signal and the phase adjusted recovered carrier must have the same phase variation going into the data detector for the best detection of I and Q channel data. The data detector has a first mixer, which uses the carrier and the phase adjusted recovered carrier to detect the I channel data; a second mixer and a phase shifter, where the phase adjusted recovered carrier is fed to the second mixer via the phase shifter, so that the second mixer uses the carrier and the phase shifted, phase adjusted, recovered carrier to detect the Q channel data. The data detector also has a first post detection filter, the output of the first mixer being passed through the first post detection filter to reproduce the I channel data, and a second post detection filter, the output of the second mixer being passed through the second post detection filter to reproduce the Q channel data.
In a further aspect of the present invention, a method for demodulating a carrier includes steps of down converting the carrier to produce a frequency shifted signal; feeding the frequency shifted signal to a phase locked loop; using the phase locked loop to produce a recovered carrier, where the recovered carrier has a frequency approximately equal to that of the carrier; adjusting the phase of the recovered carrier to produce a phase adjusted recovered carrier; providing the carrier and the phase adjusted recovered carrier to a data detector; and demodulating the carrier and detecting I channel data and Q channel data using the data detector.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.